1. Technical Field
The present invention relates to a narrow-band discharge excited excimer laser device and, in particular, to a narrow-band discharge excited excimer laser device for use in exposure devices, which is capable of high output, realizing high band-narrowing efficiency, and oscillating at a high repetition rate.
2. Related Art
It is believed that, as the technology node is further scaled down, for example for the technology nodes of 45 nm to 32 nm, double exposure techniques such as double patterning and increased NA (1.3 to 1.5) using immersion technique will become principal techniques for exposure devices using an ArF excimer laser as a light source.
The following are requirements for light sources for ArF laser exposure devices:
1. A high repetition rate (10 kHz or higher) and high average output (100 W or higher) are required to ensure high dose stability and to increase throughout;
2. Increased NA requires further narrowing down of super narrow band (spectral width of 0.3 pm or less at a spectral purity of 95%);
This requirement is due to miniaturization of worked patterns. In the present state of technology, a spectral line width of ArF laser is 0.5 to 0.35 pm at a purity of 95%. This trend of band-narrowing will be further enhanced in the future, and eventually a bandwidth of 0.2 pm or less will be required; and
3. The space coherence of output laser light is required to be low enough to reduce the effects of speckles on masks of the exposure devices.
In particular, for the double exposure technique which requires a wafer to be exposed twice, light-source output must be high in order to improve the productivity.
In order to satisfy these requirements, ArF excimer lasers used as a light source employ a double-chamber system in which two chambers are provided. This is because it is practically very difficult for a laser employing a single-chamber system to increase its output while maintaining desirable optical performance such as narrow spectral width, due to various restrictions relating to stable system operation, lifetime of modules and the like.
The double-chamber system is able to overcome the above-mentioned difficulty in practical application and to satisfy the industrial requirements as described above, by generating a laser beam having high optical performance with low output in a first chamber, referred to as oscillation stage, and amplifying the laser beam in a second chamber referred to as amplification stage.
The double-chamber systems are roughly classified into two types: MOPA (master oscillator power amplifier) type having no resonator mirror provided on the amplifier side, and MOPO (master oscillator power oscillator) type having a resonator mirror provided.
US Patent Application Publication No. 2002/0154668 describes a MOPA-type laser device.
The laser device described in US Patent Application Publication No. 2002/0154668 has a band-narrowing module provided in an oscillation stage laser (MO) for narrowing the band, so that laser light having a very narrow spectral width is output. This seed light is injected into an electric discharge area of an amplifier (PA) chamber to amplify the power, whereby a super narrow band and high output are realized.
WO 2004/095661 pamphlet proposes a MOPO-type laser device, in which seed light from an oscillation stage laser (MO) is injected into a low-coherence resonator of an amplification stage laser (PO).
Employing the low space coherence MOPO system, this laser device realizes higher amplification efficiency and a greater pulse width compared to the MOPA system, with the beam grade being kept equivalent to that of the MOPA system.
Examples of configuration of two-stage laser devices employing the double-chamber system are shown schematically in FIGS. 12A and 12B. FIG. 12A shows the MOPA system, and FIG. 12B shows the MOPO system.
In FIGS. 12A and 12B, a laser beam emitted by an oscillation stage laser (MO) 100 functions as a seed laser beam, and an amplification stage laser (PA) 200 or an amplification stage laser (PO) 210 has a function to amplify the seed laser light. Specifically, spectrum characteristics of the laser device as a whole are determined by spectrum characteristics of the oscillation stage laser 100, and laser output (energy or power) of the laser device is determined by the amplification stage laser 200 or 210.
In the MOPA-type laser device shown in FIG. 12A, the oscillation stage laser (MO) 100 and the amplifier (PA) 200 have laser chambers 101 and 201, respectively, which are filled with a laser gas. A pair of electrodes (not shown), which face each other and are spaced apart by a predetermined distance, are arranged within each of these chambers, so that electric discharge is generated by applying a high-voltage pulse to the pair of electrodes.
Further, window members (not shown) made of a material having transparency to laser oscillation light are provided in each of the chambers of the oscillation stage laser 100 and amplifier 200. A cross-flow fan (not shown) is disposed in the chambers 101, 201, so that the laser gas is circulated in the chambers 101, 201 to supply the laser gas to the electric discharge areas.
The oscillation stage laser 100 has a band-narrowing module (LNM) 300 composed of a magnifying prism 301 and a grating (diffraction grating) 302. A laser resonator is formed by the optical elements in the band-narrowing module 300 and a front mirror 102.
A laser beam (seed laser beam) from the oscillation stage laser 100 is guided and injected into the amplifier (PA) 200 through a beam propagation system 400 including a reflecting mirror. The laser beam is amplified in the amplifier (PA) 200 and output.
The amplifier (PA) 200 has no resonator mirror in the MOPA-type laser shown in FIG. 12A, whereas the amplification stage laser 210 in the MOPO-type laser shown in FIG. 12B is provided with a optically stable resonator composed, for example, of a rear mirror 211 and a front mirror 212 so that amplification is possible even with a low input. An injected seed laser beam is reflected between the front mirror 212 and the rear mirror 211 as indicated by the arrows, passing effectively through the electric discharge area to increase the power of the laser beam, and laser light is output from the front mirror 212.
As mentioned in the above, one of the requirements for a light source for ArF laser exposure devices is narrow spectral line width. In the present state of technology, the spectral line width of ArF laser is 0.5 to 0.35 pm at a purity of 95%. It is believed that this trend of band-narrowing will be further enhanced and eventually a width of 0.2 pm or less will be required.
However, further narrowing of the bandwidth will induce deterioration of the oscillation efficiency.
Further narrowing down of the currently available spectral purity width of 0.5 to 0.35 pm is required by the semiconductor exposure process as described above. However, if the bandwidth is further narrowed without improvement in the technology, the oscillation efficiency, particularly the band-narrowing efficiency of the excimer lasers will be deteriorated. The term “band-narrowing efficiency” as used herein means a ratio between a laser output when a band-narrowing module (LMN) is provided and a laser output when a high reflection mirror is provided instead of the band-narrowing module (LMN). Specifically, the band-narrowing efficiency is represented by the following equation.[Band-narrowing efficiency]=[laser output when LNM is provided]/[laser output when high reflection mirror is provided]
This is for the reasons as follows. The band narrowing of an excimer laser is performed by the use of a band-narrowing module. The band-narrowing module is provided therein with a magnifying prism and a wavelength dispersion element (a diffraction grating is used in the figure), which together narrow the bandwidth of the excimer laser.
The wavelength dispersion element returns only light having a specific range of wavelength to the laser chamber to thereby amplify the light and narrow the bandwidth. In order to obtain light with a narrower spectral width, the beam magnifying power of the prism must be increased. However, if the beam magnifying power is increased without changing the length of the diffraction grating, the discharge width usable for amplification will become smaller. Accordingly, if the magnifying power is increased for further narrowing the bandwidth with the discharge width being left wide, it will impair the energy efficiency in the conversion from electric discharge input to laser. This is because the band-narrowing efficiency is deteriorated.
Conventionally, the diffraction grating length has been made greater as a measure for furthering the band narrowing while preventing the deterioration of the band-narrowing efficiency. However, it is difficult to produce a diffraction grating having a greater length than those currently available since the diffraction gratings are very precise components. Further, use of a longer diffraction grating will induce problems such as increased size of the band-narrowing module. Accordingly, it is difficult to produce a laser having a high band-narrowing efficiency while furthering the band-narrowing.
The excimer laser is a so-called pulse laser emitting light intermittently. Therefore, the laser output is represented as a product of energy per pulse and repetition rate. Accordingly, two different methods are conceivable for increasing the light source output in exposure devices.
One of them is a method of increasing the light source energy. The other is a method of increasing the repetition rate. Both these methods have advantages and disadvantages. It is determined which method is to be employed, depending on the difficulty in practice or the like.
One of the main problems in such methods is durability of optical elements. In an exposure device using deep ultraviolet light, optical elements thereof are highly likely to be damaged by light since deep ultraviolet light has high photon energy. In order to avoid such damage, the optical elements must have a high durability performance, and various measures are taken for that purpose.
It is a matter of course that the durability performance of the elements themselves need be improved, while measures are also taken to reduce the load to the elements. It is known that the damages to the optical elements are reduced by decreasing the period of time and the peak strength at the position of incidence of light on the optical elements.
Accordingly, the method of increasing the light source output by increasing the pulse energy increases the load to the optical element, and hence is not preferable in view of the lifetime of the optical elements. It is therefore believed that the method of increasing the laser output by increasing the repetition rate is more preferable in view of increasing the lifetime of the optical elements and hence of the exposure system as a whole and building up a stable system.
FIG. 13 is a cross-sectional view of a laser chamber of a discharge excited laser device.
The laser chamber 100 includes a cross-flow fan 121 for supplying flow of a laser gas between electrodes, a heat exchanger 122 for cooling the laser gas after electric discharge, an anode electrode 131 and a cathode electrode 132 for discharge excitation, and an air guide 123 for efficiently supplying the laser gas fed by the cross-flow fan 121 to between the electrodes.
An electric discharge space is pre-ionized by applying a voltage to a pre-ionization electrode 125 for a short period of time by a power supply. Then, electric current is caused to flow from the cathode electrode to the anode electrode to generate electric discharge.
An insulating ceramics 124 is additionally provided between the power supply 133 and the electrode 132, and the pre-ionization electrode 125 is provided in the vicinity of the electrode 131.
A voltage is applied to the pre-ionization electrode 125 by the power supply 133 to pre-ionize the electric discharge space, and then a pulse voltage is applied between the electrodes 131 and 132 to cause electric current to flow between the cathode electrode 132 and the anode electrode 132, thereby causing electric discharge to occur.
Light generated by the electric discharge and having a specific wavelength is selected and amplified by a resonator, whereby excimer laser is oscillated.
When the excimer laser is to be operated at a high operating frequency, discharge stability becomes a problem. In the excimer laser, as described above, the laser gas is excited by generating intermittent electric discharge so that the laser is oscillated. This intermittent discharge, however, cannot be obtained stably if the repetition rate is increased. It is believed that this is because when the repetition rate is increased, the electric discharge will change from normal glow discharge to arc discharge or streamer discharge, breaking the uniformity of the laser gain in the electric discharge, and making it impossible to ensure a gain length required for laser oscillation.
In order to increase the repetition rate in the excimer laser, it is most effective to narrow the discharge width. The maximum operable repetition rate in excimer lasers is usually explained in association with a clearance ratio (CR). The term CR refers to a ratio between a discharge width W and a product of a gas flow rate between electrodes represented by v and discharge interval time represented by t, that is, CR=vt/W. When the CR value is sufficiently great, the electric discharge will be stable and the laser is allowed to operate at a high repetition rate. If the CR is great, the electric discharge can be generated stably, and the stability in laser energy is also improved.
The CR can be explained as follows, based on physical phenomena.
Discharge products produced by electric discharge, such as ions and active species, and dust and debris derived from the electrodes remarkably reduce the discharge resistance of the gas. In addition, the electric discharge will produce a dilute gas region where the gas pressure is relative low and hence the discharge resistance is low. Accordingly, if such dilute-gas region (or discharge product) exists in the vicinity of the electrodes, the next electric discharge will be generated in this region instead of between the electrodes. Consequently, when the CR is great, it means that the discharge product is kept away from the vicinity of the electrodes during the occurrence of the next electric discharge. FIG. 14 is cross-sectional view of the vicinity of the electrodes, schematically illustrating normal electric discharge and abnormal electric discharge. In FIG. 14, the reference numerals 131 and 132 denote electrodes, and G denotes an inter-electrode gap.
A necessary CR value differs depending on usage of the laser. A CR of about two will suffice in an application where the level of the energy stability required for the laser is not so high. However, when the laser is used as a light source for semiconductor exposure, a high level of energy stability is required, and hence a CR of three or higher is necessary. Accordingly, if the laser is to be operated at 6 kHz with the discharge width being set to 3 mm, for example, the gas flow rate between the electrodes has to be about 50 m or more per second.
A description will be made of the double chamber system mentioned above. In the double-chamber system, the oscillation stage chamber encounters less technical difficulty in increasing the repetition rate in comparison with the amplification stage. This is because the output or energy required for the oscillation stage is generally low, and the oscillation stage can be designed to set low the energy density of output beams. If the energy density of output beams is high, it will damage the optical elements such as output mirrors and output windows, making it impossible to achieve the performances required in practice including the lifetime. The narrowing of the discharge width will reduce the width of a laser beam. Therefore, if the discharge width is narrowed to try to keep the same output energy, the energy density will be increased. An allowable discharge width for the oscillation stage is small, since the output energy is low. Conversely, an allowable discharge width for the amplification stage is apt to be great since the output energy is high. In order to avoid this, some double-chamber systems employ two amplification stage chambers. This is because a twice higher frequency can be achieved by alternately operating the two amplification stage chambers. For example, operation at 10 kHz is possible when the oscillation stage is operated at 10 kHz, while the two amplification stage chambers operating at 5 kHz are operated alternately.
One of possible measures for increasing the energy density is to set the inter-electrode distance great. Even if the beam width is narrowed, the cross section of a laser beam can be enlarged by setting the beam height greater by that much, and the energy density can be reduced. Conventionally, high repetition rate lasers have been designed in this manner.
FIG. 15 schematically shows a configuration example of a MOPA-type laser device employing two amplifiers (PA) described in U.S. Pat. No. 7,006,547. FIG. 15A is a side view and FIG. 15B is a top view of the amplifiers (PA).
The laser device shown in FIGS. 15A and 15B is a MOPA-type laser device which includes a single high-repetition oscillation stage laser (MO) and at least two amplifiers (PA) (several pairs of electrodes may be arranged in one and the same chamber), and is designed such that synchronous operation is achieved between the oscillation stage laser (MO) operating at a repetition rate, for example, of 4 kHz or higher and the amplifiers (PA) operating at a repetition rate, for example, of 2 kHz or higher
In FIGS. 15A and 15B, the repetition rate of the oscillation stage laser 100 is for example 4 kHz or higher, and a laser beam 140A from the oscillation stage laser 100 is injected into the amplifier (PA) 200 via reflecting mirrors 240A and 240B.
The amplifier 200 is provided with two pairs of discharge electrodes 90A-92A and 90B-92B. These electrode pairs alternately generate electric discharge at a repetition rate for example of 2 kHz or higher. The injected laser beam is reflected by the reflecting mirrors 240B, 240C1, and 240C2 as shown in FIG. 15B to be amplified. The amplified laser light is output from the amplifier 200.
As mentioned in the above, a spectral line width of 0.2 pm or less at a purity of 90% will be required in the future.
As shown in FIGS. 12A and 12B and others, the band narrowing is carried out by means of a band-narrowing module (LNM). The LNM has a prism for expanding a laser beam and a diffraction grating for selecting a wavelength. When the spectral line width is to be narrowed, it is a common practice to employ a method for improving the wavelength dispersion of the diffraction grating, a method for increasing the laser beam magnifying power, or the like.
Although the grating density of the diffraction grating must be increased in order to improve the wavelength dispersion of the diffraction grating, it is very difficult to fabricate such a diffraction grating. Further, if the laser beam magnifying power is increased, the band-narrowing efficiency will be deteriorated unless the length of the diffraction grating is increased. However, it is difficult to fabricate a large-sized diffraction grating with high accuracy.
In the present state of technology, the practical limit of the length downsizing of a diffraction grating is about 350 mm when a diffraction grating having desirable reflection efficiency for the ArF wavelength (193 nm) is to be fabricated with high accuracy. If the laser beam magnifying power is increased without changing the length of the diffraction grating, the spectral line width is narrowed but the band-narrowing efficiency is deteriorated. This is because the width of the gain region used for laser oscillation is relatively narrowed by increasing the laser beam magnifying power. For example, when the magnifying power is doubled without changing the length of the diffraction grating, the spectral line width is reduced to two thirds, whereas the width of the gain region used for laser oscillation will be substantially halved and the output will be reduced to about a half.
Conventionally, the effort to narrow the bandwidth has been pursued by sacrificing the band-narrowing efficiency. However, the further band narrowing is possible without deteriorating the band-narrowing efficiency, if the width of the electrodes arranged in the laser chamber is reduced so that the discharge width, or the gain width is concentrated in the narrowed region. The concentration of the electric discharge to the narrowed region makes it possible to improve the band-narrowing efficiency by reducing the unusable region even when the band is further narrowed.